Over the years, one of the most persistent problems encountered in the oil field industry has been the problem of correctly judging when a connection has been properly assembled between successive sections of drill pipe, tubing, casing or the like. Presently, such tubing, casing, and drill pipe connections are assembled on the basis of a specified torque beyond hand tightness, either in accordance with American Petroleum Institute specifications, or torque values supplied by the company whose connectors are being used. Generally, male threaded members are attached to the two sections of pipe or tubing to be joined together. The female threaded member, also called the coupling or box member, is then attached, and the connection is tightened to a hand-tight condition. In most cases, power tongs are then applied to tighten the connection until the recommended or specified torque value is reached.
If the connection is not torqued enough, insufficient radial thread interference in the connection may result in reduction of the pressure capability of the seal. This can cause leakage if the bearing pressure between the mating surfaces is less than the internal pressure encountered in the particular well.
Application of too much torque to the connection may cause either member to exceed its yield point, thereby resulting in a loss of its dimensional accuracy. When this happens, the connectors are not interchangeable and their use may create a hazard in the well.
Several apparatus have been devised for controlling the application of torque to such connections. For example, U.S. Pat. No. 4,125,040 discloses a power tong apparatus which includes an automatic shut off capability when a predetermined torque has been achieved. Additional apparatus and methods are described and referenced in that patent.
The use of torque as a measure of the proper stress level in either the male or female members of a threaded connection does not result in attainment of the actual stress level in such a connection. The relationship between torque and stress is tied to a friction factor, which cannot be accurately determined due to fluctuating variables such as the type of lubrication used on the connection, temperature, surface finish of the threads, machining tolerances, etc. Hence, torque is at best a "ball park" way of determining the proper makeup of a threaded connection. In particular, such a method does not indicate the proper makeup of the connection if the taper of the male member, also called the pin, is over tolerance, and/or the box member is under tolerance. Although the connector may meet the specified torque requirements due to a tight fit at one end of the female member, it may not hold as much pressure as it should, if the overall tolerance of the female member exceeds specified limits. In addition, overtorque can result if the recommended or specified torque value has not been accurately determined. This causes deformation of the female member and loss of interference in the connection.
The recommended torque used in actual practice is calculated using a formula derived from tests conducted under controlled laboratory conditions. If any of the variables in the formula change, then the torque requirements change. Since it is impractical to determine each variable in the field and correspondingly adjust the recommended torque, the connections are torqued to the nominal torque based on perfect conditions. In the laboratory, strain gauges may be used to relate the hoop stress to torque. However, it is not possible to separate out defects in diametrical tolerance, taper tolerance, thread crest height tolerance and lead tolerance using standard thread hardened and ground ring and plug gauges.
Most importantly, conditions approximating those in the laboratory very rarely occur in the field. If problems develop in the connectors, they are reworked in field shops or in whatever machine shop facility is available, with little or no access to appropriate gauges. This leads to incorrect tapers, gauge standoffs and other dimensional inaccuracies. Such connectors are torqued up in the field to torques that are based on nominal tapers and gauge standoffs, thereby leading to incorrect stress in the female member. Inaccurate tapers result in unpredictable radial thread interference which may cause a loss of pressure containment of the assembled connection. In drill pipe, abnormal hoop stress can cause failure due to fatigue.
At the present time there is no practical way to determine hoop stress at the outer diameter of a female connector member in the field. Strain gauges and torque formulas may be useful in the laboratory, but have proven unworkable in field use. When a failure occurs, a field engineer may advise on the use of more or less torque to prepare the connection. However, the failure which resulted in adjustment of the torque may have already resulted in delays in the project, and additional expenses.
Accordingly, it is a primary object of this invention to properly gauge the hoop stress in a female threaded connector member caused by tightening of a tapered male threaded member into the female member.
It is a further object of this invention to detect undesirable tapers in the threads of a male or female threaded connector member in the field.
Another object of the invention is to provide a threaded connection which will have the lowest possible stress at the maximum sealing integrity.
Additional objects and advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.